A wellhead is equipped with a gate valve in which a piston-like valve actuator biases the valve to the closed position by a spring acting against the lower surface of the actuator piston head. Pneumatic pressure engages the opposite surface of the piston to overcome the biasing spring and hold the valve in the open position. In the event of a well fire, pressure to the actuator is vented and valve body pressure, aided by the biasing spring, automatically shuts the valve, stopping flow of well fluids, thereby preventing an uncontrolled continuous flow of fuel to the fire.
Events causing loss of actuator pressure are not uncommon. Such an inadvertent loss which automatically closes the gate valve, at time, can be expensive, time-consuming and frustrating. One such time is during the performance of routine maintenance or well testing using conventional wireline techniques. A loss of actuator pressure closing the gate valve severs the wireline, dropping the tool being employed, along with several thousand feet of wireline, down the well. In addition to any damage that may occur to the tool, additional time and expense are incurred in fishing the wireline and tool out of the wellbore resulting in both the cost associated with the manhours of labor and the hours of lost well production.
In order to avoid such expensive fishing expeditions, it is a common practice to mechanically disable the biasing spring (i.e., to jam the actuator open) during wireline operations to guard against the possibility of an inadvertent valve closure. Such a practice is obviously unsafe and represents a significant gamble that a fire will not occur during the time when the valve is jammed open.
In response to customer reaction to this problem, valve manufacturers began manufacturing a "fusible lock-open device" that enabled the valve to be held open by a cap assembly over the end of the actuator shaft. This cap had a portion made of fusible material which, in the event of a fire, melted allowing the valve to close. While these modified valves solve the safety problem, they do not allow the valves to operate in their normal modes without removing the cap assembly. When special operations are needed or required, such as using a wireline to drop a tool down the well, the valve must be forced into the open position and then the cap assembly installed on the outside of the valve actuator to engage the actuator shaft. If the pressure is lost to the valve actuator, the cap assembly holds the valve open and provides the fire protection. When the operation is completed, the cap assembly must be removed for normal valve operation to resume.
Additionally, manufacturers of wellhead gate valves do not produce a standard length actuator shaft and, therefore, once the valve is open, the length of shaft protruding from the gate valve housing is not always the same. This requires a need for a retrofit cap assembly to be adjustable for various length actuator shafts.
Moreover, there are situations where a customer would want to close the wellhead valve before the fusible material melted, such as when a fire is anticipated. There occurs situations where closing the valve is needed quickly and preferably from a remote location.
It is, therefore, an object of the present invention to provide an improved fusible lock-open cap assembly for attachment to an actuated gate valve assembly to hold the valve in the stroked position.
Another object of the present invention is to provide a fusible lock-open cap assembly that is adjustable to fit with gate valves having various length actuator shafts.
It is a further object of the invention to provide a fusible lock-open cap assembly having a quick-release mechanical feature to enable the valve to operate in its normal mode without removing the cap assembly from the valve. It is a related object of the invention to provide a fusible lock-open cap assembly having the quick-release mechanical feature remotely operable in the event of an emergency.